Re: [PATCH] c++: Quash -Wdangling-reference for member operator* [PR107488]

[Marek Polacek <polacek@redhat.com>]

On Thu, Nov 03, 2022 at 02:54:12PM -0400, Jason Merrill wrote:
> On 11/1/22 18:06, Marek Polacek wrote:
> > -Wdangling-reference complains here:
> > 
> >    std::vector<int> v = ...;
> >    std::vector<int>::const_iterator it = v.begin();
> >    while (it != v.end()) {
> >      const int &r = *it++; // warning
> >    }
> > 
> > because it sees a call to
> > __gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator*
> > which returns a reference and its argument is a TARGET_EXPR representing
> > the result of
> > __gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator++
> > But 'r' above refers to one of the int elements of the vector 'v', not
> > to a temporary object.  Therefore the warning is a false positive.
> > 
> > I suppose code like the above is relatively common (the warning broke
> > cppunit-1.15.1 and a few other projects), so presumably it makes sense
> > to suppress the warning when it comes to member operator*.  In this case
> > it's defined as
> > 
> >        reference
> >        operator*() const _GLIBCXX_NOEXCEPT
> >        { return *_M_current; }
> > 
> > and I'm guessing a lot of member operator* are like that, at least when
> > it comes to iterators.  I've looked at _Fwd_list_iterator,
> > _Fwd_list_const_iterator, __shared_ptr_access, _Deque_iterator,
> > istream_iterator, etc, and they're all like that, so adding #pragmas
> > would be quite tedious.  :/
> 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> OK.

Thanks.
 
> It also occurred to me that we should avoid warning if the reference we're
> initializing is a non-const lvalue reference, which can't bind to a
> temporary.

Yup; amusingly I noticed that too while working with the reduced version
of the testcase, which I deliberately didn't end up using, and which
reduced to 'int&' rather than 'const int&'.

> Maybe also if the function returns a non-const lvalue reference.

Ok.  Expect a patch soon.

Marek